1. Field of the Invention
The present invention is generally in the field of semiconductor circuits. More specifically, the present invention is in the field of transmission line impedance control in semiconductor circuits.
2. Background Art
Consumer demand for smaller, more complex, and faster devices operating at high frequencies, such as wireless communications devices and Bluetooth RF transceivers, has resulted in an increased demand for semiconductor dies with higher densities. As the density of the semiconductor die increases, the number of input/output (xe2x80x9cI/Oxe2x80x9d) pads on the periphery of the semiconductor die also increases. As a result, there is a diminishing amount of space on the periphery of the semiconductor die in which to place the I/O pads. To solve this problem, semiconductor manufacturers are moving to area array I/O designs.
In an area array I/O design, I/O pads on the periphery of the semiconductor die are relocated to an area array in the center of the semiconductor die using a redistribution layer (xe2x80x9cRDLxe2x80x9d). Typically, the RDL is a thin film routing layer formed primarily from aluminum or copper. The RDL comprises interconnect lines, also referred to as transmission lines in the present application, that provide electrical connections between I/O pads on the periphery of the semiconductor die and solder bumps in an area array configuration. Under bump metal (xe2x80x9cUBMxe2x80x9d) is used to attach the solder bumps to the interconnect lines.
The interconnect lines, i.e. the transmission lines, in the RDL are driven by I/O drivers (or xe2x80x9cline driversxe2x80x9d) located near the I/O pads in the semiconductor die. The I/O drivers expect to interface with transmission lines having a certain impedance. If the impedance of a transmission line does not match the impedance of its respective I/O driver, the transmission of electrical signals through the transmission line will not occur in an optimized manner and will be generally slowed down.
One approach used to reduce the impact of this impedance mismatch problem has been to increase the strength of the I/O driver. However, utilization of substantially stronger I/O drivers will increase the cost of manufacturing the semiconductor die. Another approach used to address the problem has been to increase the number of I/O drivers that drive the transmission line. However, increasing the number of I/O drivers also increases the cost of manufacturing the semiconductor die.
Thus, there is a need in the art for a transmission line in an RDL that can match a required impedance of an I/O driver in a semiconductor die.
The present invention is directed to controlled impedance transmission lines in a redistribution layer. The invention overcomes the need in the art for transmission lines in a redistribution layer that can match required impedances of I/O drivers in a semiconductor die.
In one embodiment, the invention includes first and second transmission lines fabricated in a redistribution layer over a semiconductor die. The first transmission line has a first distance from a first ground return path formed in a first metal level. For example, the first ground return path might be formed in metal level six within the semiconductor die. The first transmission line has a first impedance corresponding to the first distance. In other words, the impedance of the first transmission line is affected by the distance between the first transmission line and the first ground return path, which in this example is the distance between the first transmission line and the ground return path formed in metal level six.
Similar to the first transmission line, the second transmission line has a second distance from a second ground return path formed in a second metal level. For example, the second ground return path might be formed in metal level five within the semiconductor die. The second transmission line has a second impedance corresponding to the second distance. In other words, the impedance of the second transmission line is affected by the distance between the second transmission line and the second ground return path, which in this example is the distance between the second transmission line and the ground return path formed in metal level five.
Thus, by determining which particular metal level in the semiconductor die should serve as a ground return path for a particular transmission line, the invention results in an additional control over the impedance of transmission lines, beyond the conventional control over the impedance which consisted of controlling parameters such as width and thickness of the transmission lines.
Various other concepts, features, and advantages of the present invention are discussed in the following sections of the present application.